There are various known multi-channel LED light sources. One possible arrangement makes use of different color channels in parallel. Each channel may for example independently provide a different color output. Alternatively, different LEDs may be provided in series, and bypass switches can be used to select which LEDs are activated, and thereby control the output color.
This invention relates in particular to the use of multiple channels in parallel.
Such systems face a major problem of limited space assigned for the drivers. For example, these systems may generate white light by driving red, green and blue LEDs independently. Note that in practice, a green LED may make use of a native blue LED and a green phosphor layer.
Systems of this type can also be used to generate white light with different color temperatures, for example having separate LED strings to generate cold white or warm white from a single luminaire. Alternatively, such systems can provide full output color control.
In addition, multi-channel LED drivers are also encountered in LED modules or LED luminaires in which different channels are used to generate separate beams for general lighting and task lighting.
In current implementations, the system requires separate drivers for the different LEDs of the module. For color tunable lamps for example, multiple LED channels are required in the driver to control the intensity of the different base colors. The intensity can be controlled by variation of the (continuous) currents or controlling the “on” time of the different colors using pulse width modulation (PWM) in each string. The PWM solution is preferred because of the more complicated requirements of current control.
Separate drivers may be needed for example as a result of the different load dependencies of the different channels. A problem arises because the available space for the light source drivers is fixed to meet the requirements of traditional light sources, which normally comprise one or at most two channels, with limited functions such as a dimming function. Multi-channel light sources with warm white and cool white channels, RGB channels, or more channels have a total peak power as well as a total space consumption which is the combination of the requirements for each channel. In order to compress the driver into a small space, basic performance has to be sacrificed, such as the power factor or efficiency, but this is generally not acceptable to the product designer. There is therefore a need to enable miniaturization of the driver circuits, without compromising the system performance.
FIG. 1 shows a conventional multi-channel lighting system driver circuit. Three LED loads 10,11,12 are shown, which may for example have three different color outputs. Each is driven by a respective driver 20,21,22 which essentially comprises a switch mode power supply (SMPS) or linear driver which implements PWM control. There is a global AC-DC converter 14, which includes power factor correction, and a global controller 16 which is remote to the actual light sources themselves. The global controller 16 provides commands to the local drivers 20,21,22 to control the operation of the LED loads.
This approach has a two-stage driver concept. One driver stage is to convert the mains voltage to an intermediate direct voltage and the other is to convert the intermediate voltage to a LED current. The multiple LED channels are then controlled independently from each other. For this two-stage driver, topologies are available to control multiple LED channels more or less independently from each other.
A single stage driver concept can be chosen to reduce costs. However, with such a topology, it is difficult to control multiple color LED channels. By-pass switching as used for color tunable lamps cannot be used because the buffer capacitor will be de-charged every time a switch is closed. With channels in parallel, the resulting system has high dependencies between the channels.
There are white-only lamps which implement color temperature adjustment as a function of dimming level. This dependency of color on the dimming level is termed a “dimtone” feature or function in the description below. This feature provides a color temperature which becomes warmer at lower dimming levels thereby mimicking the behavior of incandescent bulbs. However, with a single stage driver topology, which drives the full array of connected LEDs, it is difficult to control the multiple color temperature LED channels. If the multiple channels are in parallel, the resulting system will have high dependency between the channels.
Solutions that are used to implement the dimtone feature in non-connected LEDs cannot be used for connected lamps driven by a single driver stage.
Controlling the LED currents in the different parallel LED channels is important to set the correct color point in these types of tunable LED lamps. This problem becomes larger when also the dimming state of the lamp needs to be controllable separately from the color point.
There is therefore a need for a method to properly control both a single stage driver as well as the PWM switches of individual LED channels to ensure that the correct color point and dimming state (i.e. brightness level) can be set.
US 2016/0088697 A1 discloses a circuit for driving a light source including a power converter coupled between a power source and the light source, and a controller coupled to the power converter. The power converter receives power from the power source and provides a regulated power to the light source. The controller receives a conduction status signal indicating a conduction state of a dimmer coupled between the power source and the power converter, and adjusts the brightness of the light source based on the conduction status signal. The controller also receives an operation indicating signal indicative of operation of an ON/OFF switch coupled to the dimmer, and adjusts color temperature of the light source based on the operation indicating signal.